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3Astronomy & Astrophysicsmanuscript no. aa201220218 c© ESO 2018August 17, 2018

Discovering young stars in the Gum 31 region with infraredobservations⋆

H. Ohlendorf1, T. Preibisch1, B. Gaczkowski1, T. Ratzka1, J. Ngoumou1, V. Roccatagliata1, and R. Grellmann1

Universitats-Sternwarte Munchen, Ludwig-Maximilians-Universitat, Scheinerstr. 1, 81679 Munchen, Germanye-mail:ohlendorf@usm.uni-muenchen.de

Received 14 August, 2012; accepted 24 January, 2013

ABSTRACT

Context. The Gum 31 bubble containing the stellar cluster NGC 3324 is apoorly-studied young region close to the Carina Nebula.Aims. We are aiming to characterise the young stellar and protostellar population in and around Gum 31 and to investigate the star-formation process in this region.Methods. We identify candidate young stellar objects fromSpitzer, WISE, andHerschel data. Combining these, we analyse thespectral energy distributions of the candidate young stellar objects. With density and temperature maps obtained fromHerschel dataand comparisons to a ‘collect and collapse’ scenario for theregion we are able to further constrain the characteristicsof the region asa whole.Results. 661 candidate young stellar objects are found from WISE data, 91 protostar candidates are detected throughHerschelobservations in a 1.0◦ × 1.1◦ area. Most of these objects are found in small clusters or arewell aligned with the H II bubble. We alsoidentify the sources of Herbig-Haro jets. The infrared morphology of the region suggests that it is part of the larger Carina Nebulacomplex.Conclusions. The location of the candidate young stellar objects in the rim of the H II bubble is suggestive of their being triggeredby a ‘collect and collapse’ scenario, which agrees well withthe observed parameters of the region. Some candidate youngstellarobjects are found in the heads of pillars, which points towards radiative triggering of star formation. Thus, we find evidence that inthe region different mechanisms of triggered star formation are at work. Correcting the number of candidate young stellar objects forcontamination we find∼ 600 young stellar objects in Gum 31 above our completeness limit of about 1M⊙. Extrapolating the intitalmass function down to 0.1M⊙, we estimate a total population of∼ 5000 young stars for the region.

Key words. Stars: formation – Stars: protostars – ISM: jets and outflows– Herbig-Haro objects – ISM: clouds – ISM: bubbles

1. Introduction

The bubble-shaped H II region Gum 31 around the young stel-lar cluster NGC 3324 is located≈ 1◦ north-west of the CarinaNebula (NGC 3372; see Smith & Brooks 2008 for a recentreview). While numerous observations of the Carina Nebulahave been performed in the last few years and provided com-prehensive information about the stellar populations as wellas the cloud properties (Yonekura et al. 2005; Smith & Brooks2007; Kramer et al. 2008; Smith et al. 2010a,b; Townsley et al.2011; Preibisch et al. 2011a,b,c, 2012; Salatino et al. 2012) theGum 31 region has not received much attention. Despite its inter-esting morphology and the publicity of HST images of its west-ern rim (Hubble News Release STScI-2008-34), the H II regionand its stellar population remain rather poorly studied until to-day. This seems to be related to its celestial position: the close-ness to the extremely eye-catching Carina Nebula has alwaysovershadowed NGC 3324.

The physical relation between NGC 3324 and the CarinaNebula complex (CNC) is still unclear. Recent distance deter-

⋆ This work is based in part on data collected byHerschel, an ESAspace observatory with science instruments provided by European-ledPrincipal Investigator consortia and with important participation fromNASA, and on data observed by VISTA (ESO project number 088.C-0117(A)), an ESO survey telescope developed by a consortiumof 18universities in the United Kingdom, led by Queen Mary, University ofLondon.

minations of Gum 31 and NGC 3324 yielded values of 2.3 kpcfor the cluster NGC 3324 (Catalogue of Open Cluster Data;Kharchenko et al. 2005) and 2.5 ± 0.3 kpc for the clouds inand around Gum 31 (Barnes et al. 2010). This implies that theNGC 3324/Gum31 region is located at the same distance as theCarina Nebula (≈ 2.3 kpc; see Smith 2006c).

The diameter of the Gum 31 H II region is∼ 15′ (10 pc). ThisH II region in turn is surrounded by an expanding H I shell whichencloses it almost completely (Cappa et al. 2008).

The available information about the stellar population ofNGC 3324 is restricted to the three brightest stars. The brighteststar of the cluster is the multiple star HD 92206, which is con-stituted by two O6.5V stars HD 92206A and B and O8.5V starHD 92206C (Maız-Apellaniz et al. 2004). A further very lumi-nous star in the region is the A0 supergiant HD 92207 (= V370Car, V = 5.49). This object is a very luminous (log(L/ L⊙) =5.56) massive (Minitial ≈ 30M⊙) evolved star (age≈ 7± 1 Myr;Przybilla et al. 2006) that drives a very strong stellar wind(M =1.3 · 10−6 M⊙ yr−1; Kudritzki et al. 1999). The membership ofHD 92207 is not entirely clear: Claria (1977) and Carraro etal.(2001) assume it not to be part of the cluster. However, Forte(1976, from HD 92207 being wrapped in a nebular shell asso-ciated with Gum 31) and Baumgardt et al. (2000, from propermotions) determine it to be a member.

Carraro et al. (2001) identify 25 candidate members ofNGC 3324 with optical photometry and suggest that the clus-ter is very young (. 2 – 3 Myr). With three O-type stars (M ≥

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H. Ohlendorf et al.: Discovering young stars in the Gum 31 region with infrared observations

18M⊙), the field star initial mass function (IMF) representationby Kroupa (2002) suggests that there should be≈ 1500 low-mass (0.1 M⊙ ≤ M ≤ 2 M⊙) stars present. This implies that thevast majority of the stellar population of NGC 3324 is still un-known.

In addition to the optically visible stellar cluster in theH II region, there is a population of young stars embedded inthe molecular cloud surrounding the H II region. The stars inthis population are only seen in infrared images (Cappa et al.2008) or traced by their protostellar jets (Smith et al. 2010a).Cappa et al. (2008) sampled point sources in a region of 20′ ra-dius centred on NGC 3324 from the IRAS, MSX and 2MASSpoint-source catalogues and identify 12 (IRAS), 9 (MSX) and26(2MASS) candidate young stellar objects (cYSOs) using colour-colour criteria. Due to the limited sensitivity and angularresolu-tion of these data, the currently known few dozen of embeddedinfrared sources represent only the tip of the iceberg; manymoreembedded young stellar objects (YSOs) must be present in thisarea and waiting to be discovered.

In this paper, we are aiming to characterise the protostellarand young stellar population of the stellar cluster, the surround-ing H II region and its environs. Our study is based onSpitzer,WISE, Herschel, and VISTA data, which provide much bettersensitivity and angular resolution than the previously existingdata sets.

We will describe the data sets we used in Sect. 2. In Sect. 3,we will describe the general morphology of the infrared cloudsin and around the Gum 31 region and its implications for ourwork. Detections of cYSOs from the IR data, including the iden-tification method, and their spatial distribution will be describedin Sect. 4. Furthermore, we have analysed point-like sources asdetected with bothHerschel andSpitzer and derived their spec-tral energy distributions (SEDs; Sect. 5). We have also identi-fied likely sources to two previously detected Herbig-Haro jets(Sect. 6). Inferences will be discussed in Sect. 7.

2. Observational data

2.1. Spitzer images and photometry

2.1.1. IRAC

We are using data taken during theSpitzer cold mission phase, inJuly 2008, with the InfraRed Array Camera (IRAC; Fazio et al.2004) (“Galactic Structure and Star Formation in Vela-Carina”programme; PI: Steven R. Majewski, Prog-ID: 40791); retrievedthrough theSpitzer Heritage Archive.1 The Basic CalibratedData were assembled into mosaics using version 18.4.9 ofthe MOsaicker and Point source EXtractor package (MOPEX;Makovoz & Marleau 2005) provided by theSpitzer ScienceCentre2 (SSC). From the complete data set recorded in the ob-servation programme, we use only a 1.0◦×1.1◦ area correspond-ing to the Gum 31 region and a small region to the west of it.This complements the area as described by Gaczkowski et al.(2013) to complete the area surveyed with ourHerschel obser-vation programme (cf. Sect. 2.2).

Using the Astronomical Point source EXtractor (APEX)module of MOPEX we then performed source detection andphotometry on these IRAC mosaics. Photometry was first carriedout individually for each image in the stack and subsequentlycombined internally to provide photometry data for the entiremosaic. Sets of Point Response Functions (PRFs) available from

1 An overview of the observations used can be found in Table 1.2 http://irsa.ipac.caltech.edu/

the SSC, chosen appropriately for the time of the observations,were employed. Before the mosaics were constructed, outlierswere removed using the Box Outlier Detection method withinMOPEX and backgrounds between the tiles were matched usingthe Overlap module. Subsequently, threeSpitzer AstronomicalObservation Requests (AORs) each were merged into a singlemosaic.

PRF-fitting photometry was carried out separately for allfour IRAC bands (3.6µm, 4.5µm, 5.8µm and 8.0µm). Thisphotometric information was then combined into a single cat-alogue by a simple nearest-neighbour matching algorithm, tak-ing the 4.5µm band as reference. (We chose this band as ourreference band because it clearly is the most sensitive one andless likely to be subject to misidentifications. These are com-mon especially in the longest-wavelength bands where randomfluctuations in nebulosities are detected as point-like sources.)Thus, the resulting catalogue is rather conservative and excludesany object not detected in the 4.5µm band. On the other hand, itminimises contamination. The total number of point-like sourcesdetected in at least one band in the study area is 57 828.

We noticed a slight misalignment between theSpitzer bandsand so globally shifted the 4.5µm, 5.8µm, and 8.0µm bandpositions by−0.17′′, −0.1′′, and−0.13′′, respectively, in dec-lination with regard to the 3.6µm band that we found to bewell aligned with the Two Micron All Sky Survey (2MASS;Skrutskie et al. 2006). In the resulting catalogue, the rootmeansquare (RMS) of the spatial deviations between the IRAC andthe 2MASS position is 0.22′′.

Due to the very strong spatial inhomogeneity of the cloudemission in our maps, the sensitivity for IRAC cannot be pre-cisely quantified by a single value. Instead, we characterise it bytwo typical values, the detection limit and the typical complete-ness limit. The former is quantified by the faintest sources inour sample. For 3.6µm, 4.5µm, 5.8µm, and 8.0µm these havefluxes of 100µJy, 100µJy, 330µJy, and 280µJy, respectively.

To estimate the completeness limits, we constructed his-tograms of the measured source magnitudes (Fig. 3) and esti-mate the point where the rise in the source count is no longerwell-described by a power-law. Although it is not a formal mea-sure of completeness, the turnover in source count curves canserve as a proxy to show the typical values of the completenesslimit across the field. In this way we estimate completeness lim-its of≈ 1.5 mJy,≈ 0.7 mJy,≈ 1.2 mJy and≈ 1.6 mJy for 3.6µm,4.5µm, 5.8µm, and 8.0µm, respectively.

Comparing these to numerical models of stellar evolution(Baraffe et al. 1998), we find that for an age of∼ 3 Myr the pho-tospheric emission of a 1M⊙ star is well above the detectionlimit for WISE (Sect. 2.4) in the 3.4µm and 4.6µm bands whilefor IRAC the sensitivity extends down to∼ 0.5 M⊙. For youngerstars this boundary shifts upwards so that with WISE 1 Myr old0.5M⊙ stars are still detectable while with IRAC we could reachdown to∼ 0.25M⊙. If we compare the completeness limits to theRADMC model (Dullemond & Dominik 2004) which providescalculations of continuum radiative transfer in axisymmetric cir-cumstellar dust distributions around a central illuminating star,we find that disk masses as low as 0.013M⊙ would still be de-tectable for a 1M⊙ YSO.

2.1.2. MIPS

For the detailed analysis in Sect. 5 the IRAC data were comple-mented by photometry on a Multiband Imaging Photometer forSpitzer (MIPS) (Rieke et al. 2004) 24µm map (“Spitzer Follow-up of HST Observations of Star Formation in H II Regions” pro-

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H. Ohlendorf et al.: Discovering young stars in the Gum 31 region with infrared observations

Fig. 1. The Gum 31 region and its connec-tion to the central Carina Nebula, seen in aHerschel RGB image, with the PACS 70µmimage in blue, SPIRE 250µm in green, andSPIRE 500µm in red. The blue box marks the1.1◦ × 1.0◦ area used for analysis here. The di-agonal white line marks the border to the left ofwhich we obtained IRAC photometric data.

Table 1: Overview of observations withSpitzer andHerschel used in this work.

Instrument AOR Observation mode Observation time Date PI

Spitzer IRAC 23708160 IRAC map 3301 s 19 July 2008 S. R. MajewskiSpitzer IRAC 23699200 IRAC map 3057 s 19 July 2008 S. R. MajewskiSpitzer IRAC 23704320 IRAC map 3070 s 19 July 2008 S. R. MajewskiSpitzer IRAC 23688192 IRAC map 3082 s 19 July 2008 S. R. MajewskiSpitzer IRAC 23701504 IRAC map 3086 s 19 July 2008 S. R. MajewskiSpitzer IRAC 23695360 IRAC map 3086 s 19 July 2008 S. R. MajewskiSpitzer IRAC 23696896 IRAC map 3087 s 19 July 2008 S. R. MajewskiSpitzer IRAC 23706368 IRAC map 3088 s 19 July 2008 S. R. MajewskiSpitzer IRAC 23684352 IRAC map 3312 s 19 July 2008 S. R. MajewskiSpitzer MIPS 15054080 MIPS phot 9469 s 12 June 2006 J. HesterHerschel PACS 1342211615 SpirePacsParallel 11889 s 26 December 2010T. PreibischHerschel PACS 1342211616 SpirePacsParallel 12863 s 26 December 2010T. PreibischHerschel SPIRE 1342211615 SpirePacsParallel 11889 s 26 December 2010 T. PreibischHerschel SPIRE 1342211616 SpirePacsParallel 12863 s 26 December 2010 T. Preibisch

gramme; PI: Jeff Hester, Prog-ID: 20726) retrieved through theSpitzer Heritage Archive. The MIPS images were searched byeye for point-like sources coinciding with the previously iden-tified IRAC andHerschel (Sect. 2.2) sources. As this field isonly sparsely populated with point-like sources in the 24µmband, they were easily found by eye. MIPS has a PSF of FWHM6′′in the 24µm band, slightly degrading towards the edges ofthe 5.4′ × 5.4′ field of view, sampled onto pixels with a size of2.55′′. According to the MIPS Instrument Handbook, the 24µmsensitivity is highly dependent on the observed sky region,in anideal case it is expected to be≈ 35µJy.

Source detection and point-spread function (PSF) photome-try were then performed with StarFinder (Diolaiti et al. 2000).3

The PSF is derived as a median value from four point sources.

3 http://www.bo.astro.it/StarFinder/index.htm

It is then fit to each identified point source together with a slant-ing plane for the background and taking into account the contri-bution of bright stars adjacent to the fitting region. In thiswaythe sub-sample of MIPS point-like sources defined by IRAC andHerschel detection is analysed from the brightest (4570 mJy) tothe faintest (5 mJy) objects.

2.2. Herschel far-infrared maps

We analysed the Gum 31 region using data from our recentHerschel far-infrared survey of the CNC. These observationswere performed in December 2010 (Open time project, PI:Thomas Preibisch, Prog-ID: OT1-tpreibis-1), using the parallelfast scan mode at 60′′s−1. With simultaneous five-band imag-ing with PACS (Poglitsch et al. 2010) at 70µm and 160µm

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H. Ohlendorf et al.: Discovering young stars in the Gum 31 region with infrared observations

Fig. 2: An overview of the Gum 31 region in the optical, mid-IR, and far-IR. The ESO Wide Field Imager (WFI) optical image [ESOimage release eso1207] shows theV band in blue,R band in yellow, O III 501 nm emission in green, and Hα in red.

and SPIRE (Griffin et al. 2010) at 250µm, 350µm, and 500µmtwo orthogonal scan maps were obtained to cover an area of2.3◦ × 2.3◦. The angular resolutions of the maps are 5′′, 12′′,18′′, 25′′, and 36′′ for the 70µm, 160µm, 250µm, 350µm, and500µm band, respectively. At a distance of 2.3 kpc this corre-sponds to physical scales from 0.06 to 0.4 pc.

A full description of these observations and the subse-quent data processing is given by Preibisch et al. (2012) andGaczkowski et al. (2013). Detection and photometry of point-like sources inHerschel bands were carried out with CUTEX(Molinari et al. 2011, also used for the Hi-GAL survey), a soft-ware package developed especially for maps with complex back-ground. CUTEX calculates the second-order derivatives of thesignal map in four directions (x, y and their diagonals), allowingthe identification of point-like sources by their steep brightnessgradients. A more detailed description of the photometry processis given by Gaczkowski et al. (2013).

As any source catalogue based on maps with strongand highly spatially inhomogeneous background emission, ourHerschel source lists will miss some faint sources and at thesame time contain a small number of spurious detections. Toimprove the reliability of the source catalogue, we restrictedour analysis to those sources that are detected (independently)in at least twoHerschel maps and for the SED constructionin Sect. 5 even to those detected in at least three bands. Forthis, the point-like sources detected in each individualHerschelband were matched. The matching process was described byGaczkowski et al. (2013). In the area, we detect 91 point-likesources in at least two bands and 59 in at least three bands.For each of them we checked coincidence with IRAC-identifiedpoint-like sources. We excluded all cases where either the spatialcoincidence ofHerschel and IRAC source was not clear or wheremore than one IRAC source appeared as a possible counterpart.

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H. Ohlendorf et al.: Discovering young stars in the Gum 31 region with infrared observations

Fig. 3: Histograms of the measured source magnitudes in ourSpitzer IRAC catalogue.

This results in the 16 sources identified both inHerschel and inIRAC wavelengths that are subject to the analysis in Sect. 5.

The detection limit can be approximated by the fluxes of thefaintest detected sources; this is in the range of≈ 1 Jy to≈ 2 Jy inour maps. An estimate for the ‘typical’ completeness limit (i. e.the limit above which we can expect most objects in the surveyarea to be detected) can be derived from the histograms of thefluxes similar to those described in Sect. 2.1.1. The correspond-ing limits are at∼ 10 Jy,∼ 15 Jy,∼ 10 Jy,∼ 10 Jy, and∼ 6 Jy for70µm, 160µm, 250µm, 350µm, and 500µm, respectively.

2.3. VISTA near-IR images

An H-band image of the area around Gum 31 was obtained withthe VISTA InfraRed CAMera (VIRCAM) (Dalton et al. 2006)in the night of 15 January 2012 as the first observation of ourVisible and Infrared Survey Telescope for Astronomy (VISTA)(Irwin et al. 2004; Emerson & Sutherland 2010) survey of theCarina Nebula complex (ESO project number 088.C-0117(A)).VISTA is a 4-m class wide field survey telescope that providesa1.3◦ × 1.0◦ field of view. The near-infrared camera VIRCAMconsists of an array of sixteen individual 2048× 2048 pixelRaytheon VIRGO IR detectors, providing more than 67 mil-lion pixels with a nominal pixel size of 0.339′′on the sky andsensitive in a wavelength range of 0.85 – 2.4µm. Since the six-teen chips are non-contiguous they produce a set of sixteen non-contiguous images called a ‘pawprint’. For a contiguous skycov-

erage six pawprints, offset in x- and y-direction, are combined.The resulting ‘tile’ covers an area of 1.5◦ × 1.2◦ on the sky.

For each of the six pawprints, at five jitter positions 27 expo-sures with an integration time of 2 s each have been obtained.Asit turned out that the seeing conditions (with an average FWHMof ≈ 1.7′′) did not meet the pre-specified quality criteria, the ob-servations were terminated after the completion of this observ-ing block. The observations in the other filters and for the restof our Carina Nebula mosaic positions were successfully com-pleted a few months later and processed by the VISTA Data FlowSystem at the Cambridge Astronomy Survey Unit. However, atthe time of writing the photometric calibration of these data isstill in progress. Therefore, we use only the image data for a(preliminary) scientific analysis here, but no photometricvalues.

A preliminary photometric calibration showed that objectsas faint asH ≈ 18.5 are clearly detectable in our VISTA image.This is about four magnitudes deeper than the nominal 2MASSSurvey completeness limit for crowded locations near the galac-tic plane ofH ≈ 14.5 (Skrutskie et al. 2006).

2.4. WISE

We used catalogue data from the Wide-field Infrared SurveyExplorer (WISE; Wright et al. 2010) All-Sky Data Release(Cutri & al. 2012). These data were taken during the WISE coldmission phase from January to August 2010. We used the stan-dard aperture-corrected magnitudes obtained with apertures of

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H. Ohlendorf et al.: Discovering young stars in the Gum 31 region with infrared observations

8.25′′and corrections of 0.222mag, 0.280 mag and 0.665magfor 3.4µm, 4.6µm, and 12µm (Cutri et al. 2012). The WISEobservations have an angular resolution of 6.1′′, 6.4′′, 6.5′′,and 12.0′′ for 3.4µm, 4.6µm, 12µm, and 22µm, respectively(Wright et al. 2010). In our survey area the catalogue contains20 739 point sources with detection in at least one band. Sourceconfusion in the catalogue should not constitute a problem in theanalysis here as we required a matching radius of 0.6′′ to matchWISE to IRAC detections, the same as we used for the IRACinter-band matching. On this scale, we were able to distinguishclearly between nearest and second-nearest neighbours.

In way parallel to the way described in Sect. 2.1.1, we es-timate the detection limits to be 90µJy, 70µJy and 700µJyfor 3.4µm, 4.6µm, and 12µm (for the analysis described inSect. 4.2 we do not use the 22µm band). Analogously, we esti-mate completeness limits of 4 mJy, 3 mJy and 24 mJy for 3.4µm,4.6µm, and 12µm, respectively.

3. Morphology

Figure 1 shows the far-IR morphology of the Gum 31 bubble andits environs; the cavity of the H II region is clearly delineated.Figure 2 contrasts optical and infrared images of the Gum 31bubble. Near the centre of the bubble a cluster of stars is justdiscernible in the IRAC image. This is NGC 3324. In the MIPS24µm and PACS 70µm images the warm dust surrounding thestars of NGC 3324 forms an arc-like structure that follows theshape of the inner bubble wall.

A number of pillar-like structures extend from the edge ofthe bubble into its inner part, especially from its southernrim.Notably, some but not all optical pillars coincide with those seenin the infrared. The photodissociation regions that very sharplydelineate the edge of the bubble are well-observable in the IRAC5.8µm image where their fluorescence under the influence ofUV radiation can be seen, but also in the MIPS 24µm imagethat shows the emission from the dust grains within.

Figure 4a shows a colour temperature map for the largerGum 31 region, extending downwards into the central CarinaNebula complex. It was derived from the ratio of the PACS70µm and 160µm emission as detailed by Preibisch et al.(2012). It shows the temperatures for the H II region being com-paratively high (≈ 30 – 40 K), while those of the surroundingclouds are considerably cooler down to 20 K. The warm dustin NGC 3324 is clearly seen as an outstandingly blue (≈ 40 K)patch inside the bubble.

The G286.21+0.17 cluster (cf. Sect. 4.4.4) also stands out. Itis about 8 K warmer than its immediate surroundings. It is alsomuch denser, as can be seen from Fig. 4b which shows the hy-drogen column density derived from the colour temperaturesasdescribed by Preibisch et al. (2012). We see that the column den-sity inside the bubble is relatively low compared to its surround-ings at a few 1020 cm−2 and rises steeply by more than one orderof magnitude at the ionisation front. Other than G286.21+0.17,we see some more dense clumps scattered within the bubble rimand beyond it. A notable feature is the cluster G286.38–0.26(cf.Sect. 4.4.5) which has a column density of around 3· 1022 cm−2.In sharp contrast to the unusually warm clump G286.21+0.17,in Fig. 4a G286.38–0.26 is seen to be cooler than its surround-ings, down to≈ 20 K. The integrated cloud mass derived byPreibisch et al. is 186 700M⊙. They derive this value by inte-grating over the same column density map that we show here,but for a region around Gum 31 slightly differently defined thanthe one here.

In the longer IRAC-wavelength emission and especially inthe Herschel image, a ‘bridge’ of filamentary structure can beseen to extend from Gum 31 downwards in the direction ofthe central Carina Nebula, forming a connection. The columndensity map (Fig. 4b), too, shows that the clouds surroundingGum 31 are connected to the clouds in the more central partsof the Carina Nebula. Forte (1976) remarks that in the deep op-tical plates of Lyngå & Hansson (1972) a filamentary structureconnecting the H II region H-31 (= Gum 31; Hoffleit 1953) toNGC 3372, the central Carina Nebula, can be seen.

According to Yonekura et al. (2005), the radial velocities oftheir C18O clumps Nos. 2 – 6, which surround the Gum 31 bub-ble, range fromVLSR = −20.0 km s−1 to VLSR = −24.1 km s−1.The C18O clumps in the central and northern part of the CarinaNebula (Nos. 8 – 12) have radial velocities in theVLSR =

−25.8 . . . − 19.9 km s−1 range. This good agreement of the ra-dial velocities suggests that the clouds around Gum 31 and theCarina Nebula are connected and actually part of the same com-plex. As was argued by Preibisch et al. (2012) and in the intro-duction, we will therefore assume a distance of 2.3 kpc towardsGum 31, the same as to the Carina Nebula complex. This numberalso agrees well with recent distance determinations with inde-pendent means (see Sect. 1).

4. Young stellar objects in the Gum 31 region

4.1. Identification of YSO candidates

The selection of cYSOs in the following sections is based onthe detection of infrared excesses. It follows, therefore,that onlythose YSOs exhibiting excess infrared emission can be detectedwith these methods (cf. also Sect. 4.3). Infrared excess emis-sion is indicative of circumstellar material. This means that onlyYSOs of Class 0 to Class II are the subject of this analysis whileClass III objects must remain undetected.4

4.1.1. Selection by Spitzer colours

With IRAC data only and following the criteria established fora survey of star-forming region Pismis 24 (Fang et al. 2012;Allen et al. 2004) we were able to identify 304 infrared ex-cess sources from plotting the [3.6] − [4.5] colour against the[5.8]− [8.0] colour for those 6739 point-like sources detected inall four bands. For identification as a cYSO Fang et al. (2012)demand (Eq. 1 follows Allen et al. 2004):

[3.6] − [4.5] ≥ 0 and [5.8] − [8.0] ≥ 0.4 (1)

[3.6] − [4.5] ≥ 0.67− ([5.8] − [8.0]) × 0.67 . (2)

The resultant colour-colour diagram is shown in Fig. 5.If we also take into account the 2MASS magnitudes, we can

sample a much larger portion of sources by applying the excesscriteria of Winston et al. (2007, usingSpitzer extinction as de-rived by Flaherty et al. 2007). These yield another estimateofthe YSOs in the region from theJ − H vs. H − [4.5] colour-colour diagram. In this way, 2577 cYSOs are identified (from atotal sample of 38459 with detections in the three bands).

4 YSOs are commonly classified according to the slope of their SEDs(Lada 1987), from Class 0 protostars in the main collapse phase throughClass I YSOs shrouded in envelopes and accreting infalling circumstel-lar matter and Class II YSOs, T Tauri stars with disks, to Class III ob-jects with little or no circumstellar matter.

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H. Ohlendorf et al.: Discovering young stars in the Gum 31 region with infrared observations

(a) Colour temperature map.

(b) Map of hydrogen column density.

Fig. 4: Maps derived fromHerschel data. They show the cloud structure in and around the Gum 31 nebula and the connection to thecentral parts of the Carina Nebula (just outside the lower left edge of the maps).

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H. Ohlendorf et al.: Discovering young stars in the Gum 31 region with infrared observations

Fig. 5: Colour-colour diagram using allSpitzer IRAC bands. Thecriteria adopted for YSO identification follow Fang et al. (2012)and Allen et al. (2004).

Within this sample, it is certain that there are contaminantsmasking as YSOs. AGB stars unfortunately exhibit very simi-lar colours to YSOs and so do background star-forming galax-ies. Oliveira et al. (2009) in a spectroscopic survey of objectsdiscovered by theSpitzer Legacy Program “From MolecularCores to Planet-Forming Disks” (“c2d”) in the Serpens molec-ular cloud find a contamination of 25% within their subsam-ple of the brightest objects (F8.0µm > 3 mJy). They attributethis high proportion to the closeness of the Serpens cloud tothe Galactic plane – something which is also true for Gum 31(bGum 31 ≈ 0.2◦). We can also estimate the contamination basedon the criteria of Winston et al. (2007) for the selection of con-taminants from the [5.8]− [8.0] vs. [4.5]− [5.8] and [4.5]− [8.0]vs. [3.6] − [5.8] diagrams. This yields an estimated contamina-tion in the IRAC-selected sample of. 10%.

It is noticeable that the distribution of these cYSOs through-out Gum 31 and outside it is almost uniform. This is suspicious.We thus conclude that the contamination within this sample dueto fore- and background sources is high and do not use this sam-ple for further analysis.

4.2. Selection by WISE colours

We used WISE catalogue data to search for infrared excesssources. Koenig et al. (2012) in their study of massive star-forming regions developed a set of criteria to identify likelyYSOs from WISE four-band photometry. We applied these cri-teria accordingly.

Using the [3.4] − [4.6] and [4.6] − [12] colours, partly incombination with the [3.4] and [4.6] magnitudes, probable back-ground objects are removed from the sample before the YSO se-lection. This includes galaxies (very red in [4.6] − [12]), broad-line AGNs (of similar colours as YSOs, but distinctly fainter)and resolved PAH emission regions (redder than the majorityofYSOs). From this cleaned sample the IR excess sources are thenselected by demanding (Eqs. 3 and 4: Koenig et al. 2012)

[3.4] − [4.6] − σ1 > 0.25 and [4.6] − [12] − σ2 > 1.0 , (3)

whereσ is the quadratically added uncertainty of the respectivemagnitudes. Class I sources are a subsample of this defined by

[3.4] − [4.6] > 1.0 and [4.6] − [12] > 2.0 (4)

Fig. 6: Colour-colour diagram using three out of four WISEbands. The criteria adopted for YSO identification followKoenig et al. (2012). Probable YSOs are marked in red forClass I sources and in green for Class II.

(the rest are Class II objects). From this analysis we excluded anydata point from the catalogue that did not have a signal-to-noiseratio of 5 or better. Figure 6 shows the [3.4]−[4.6] vs. [4.6]−[12]colour-colour diagram, constructed after probable contaminatorshad been removed. The total sample before this removal were10 128 sources. After removal of probable contaminators, 6669sources remained. These were plotted in Fig. 6. Resultant Class Isources are marked in red, Class II in green. The same colour-coding is used in Fig. 8, which shows the spatial distribution ofthe cYSOs. This analysis yields 661 cYSOs of which 207 areClass I and 454 are Class II.

The distribution of WISE-selected cYSOs follows the cloudstructure as expected. Koenig et al. (2012) estimate the remain-ing contamination of the cYSO population selected with theircriteria by galaxies to be∼ 10 deg−2 which in our 1.2 deg fieldleads to an estimate of∼ 12. However, in a region projected ontothe Galactic disk as the Carina Nebula we expect a high level ofcontamination mainly from background and foreground stars. Toestimate the total contamination we performed the same analy-sis as described above for two circular areas of 30′ radius welloutside the Carina Nebula (i. e. regions that we expect to be rel-atively free of YSOs), one centred aroundα2000 = 10:51:52.5,δ2000= −59:14:30, the other aroundα2000= 10:25:10.7,δ2000=

−58:52:00. They were chosen as having a similar proximity tothe Galactic plane as Gum 31 (b = 0.15 andb = −1.18; Gum 31:b = −0.17). Furthermore, we carefully selected fields that wereas free of CO and Hα emission as possible and appeared asempty as possible in the IRAS 100µm images.

In those control fields we find a mean cYSO density of97 deg−2; with the cYSOs spread homogeneously. In the Gum 31region we determine a cYSO density of 550 deg−2. This leads usto expect a contamination of around 18% (∼ 120 cYSOs) forthe Gum 31 cYSO sample. For the IRAC data we were not ableto conduct a similar comparison as we do not have photometrydata for the full study area around Gum 31 and none for regionsoutside the central nebula.

We therefore deem the WISE-selected sample to be more re-liable and base our conclusion in the following sections predom-inantly on this sample, although we will occasionally describea classification with IRAC. In general, though, those shouldbetreated as less reliable.

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4.3. Identification of protostars from Herschel data

With the methods described in Sect. 2.2 and in more detail byGaczkowski et al. (2013), we obtained a point-source cataloguefor the Herschel data. Although we consider only ‘point-like’Herschel sources in the following, it is important to keep in mindthat the relatively large PSF corresponds to quite large physicalscales at the 2.3 kpc distance of Gum 31. In the PACS 70µmmap, all objects with an angular [spatial] extent of up to≈ 5′′

[0.056pc] are compact enough to appear ‘point-like’. For theSPIRE 250µm map, this is true for sources of up to≈ 18′′

[0.20 pc]. This shows immediately that (pre-stellar) cloudcores,which have typical radii of<∼ 0.1 pc, cannot be (well) resolvedin the Herschel maps and may appear as compact ‘point-like’sources. This implies that YSOs in all their evolutionary stagescan, in principle, appear as point-like sources in ourHerschelmaps. However, the possibility to detect an object in a spe-cific stage depends strongly on its properties: As describedbyGaczkowski et al. (2013), many pre-stellar cores (& 1 – 2M⊙)and embedded protostars (& 1 M⊙) will be easily detectable,while most of the more evolved pre-main sequence stars withdisks should remain undetected. In this last case the detectionlimit depends on the disk mass; as shown by Gaczkowski et al.a 1M⊙ YSO is still detectable if it has a disk mass of& 0.5 M⊙,this sinks to& 0.1 M⊙ for a 3M⊙ YSO and& 0.01M⊙ for a 6M⊙YSO.

This means that while with IRAC we find mainly Class IIYSOs and a number of Class I YSOs, withHerschel the em-phasis is on Class 0 protostars, with some Class I stars. Thisalso implies that the overlap between both is relatively small (cf.Sect. 5). Ragan et al. (2012) presented radiative transfer modelsof starless cores and protostellar cores and investigated the de-tectability of these classes of objects. Their models showed thatthe SEDs of starless (i. e. pre-stellar) cores typically peak around200 –300µm and drop very steeply towards shorter wavelengths.Their model fluxes at 70µm (scaled to the distance of the CNC)are several orders of magnitudes below our detection limits.Protostellar cores, on the other hand, have much stronger fluxesat PACS wavelengths. Guided by these results, we can take a de-tection at 70µm as an indication for the protostellar nature ofthe source, whereasHerschel sources without 70µm detectionwould then be pre-stellar cores. In Fig. 7 these two classes areindicated separately.

Gaczkowski et al. (2013) argue that in a sample ofHerschelpoint-like objects in the Carina Nebula it is very unlikely forcontamination by evolved stars or extragalactic objects tooccur.The same reasoning applies to Gum 31. The photometric datafor those sources that fall within the Gum 31 area can be foundin the paper of Gaczkowski et al. (2013).

4.4. Spatial distribution of the cYSOs

It is notable that both Class I and Class II sources are found pre-dominantly within the interior of the Gum 31 bubble or along itsrim. They tend to occur in clusters and in their distributionareoften correlated with the Yonekura et al. (2005) molecular cloudclumps (see below). When in the following we refer to a cYSOwith a number preceded by ‘[CNA2008]’ this source was classi-fied as a cYSO by Cappa et al. (2008).

4.4.1. NGC 3324

The cluster NGC 3324 appears prominent in theSpitzer images,containing≈ 200 point-like sources. Within the cluster itself, we

identify only a single cYSO with WISE, J103706.9–583710 atthe western edge of the cluster (Class II). From the [3.6] − [4.5]vs. [5.8]− [8.0] diagram (IRAC) we identify half a dozen cYSOscoincident with the cluster NGC 3324 and distributed about 40′′

towards the north-east of the cluster centre. The very smallfrac-tion of stars with detectable infrared excess (≈ 0.5%) supportsprevious age estimates of& 3 Myr for this cluster.

4.4.2. cYSOs in the rim of the bubble

Numerous cYSOs are found lined up along the ionisation frontof the Gum 31 bubble to the west of NGC 3324. A dozencYSOs are found right behind the edge of the ionisation front.Among them are J103653.9–583719, J103653.3–583754, andJ103652.4–583809, three Class I candidates that are found alongthe ridge traced in IRAC andHerschel images at the very edge ofthe bubble, neighbouring NGC 3324. Two of them, J103653.9–583719 and J103652.4–583809, will be discussed in Sect. 6 aspossible sources of Herbig-Haro jets.

Behind this ‘first row’ of Class I candidates, there is a ‘sec-ond row’ of five Class II candidates, all lined up about 19′′ be-hind the ionisation front. Further northwards along the rimthereare four more Class II candidates, one similarly behind the frontand three right along it.

In Fig. 8 it is evident that a major part of the cYSOs is locatedalong the bubble edges, similar to what is observed in compara-ble bubbles associated with H II regions (e. g. Dewangan et al.2012). Their distribution follows its circular shape and theirnumber sharply goes down outside the cloud structure that ap-pears magenta in the image. This is suggestive of triggered star-formation in a ‘collect and collapse’ scenario as describedbyWhitworth et al. (1994).

Following analytical models for this scenario, a stellar windbubble produced by three massive stars with spectral types O6.5,O6.5 and O9.5 could reach a radius of approximately 9 – 11 pc in1.5 – 2.0 Myr assuming initial cloud densities in a range of 500– 1000 cm−3. The radius at which fragmentation occurred wouldthen vary between 7 pc and 11 pc. To estimate the mechanical lu-minosities emitted by the three most massive stars in NGC 3324,we used values from Smith (2006a) (and the erratum (Smith2006b)) for stars with the same spectral types (Luminosity classV) observed in the Carina Nebula. The∼ 15′ diameter of theGum 31 H II region corresponds to 10 pc at a distance of 2.3 kpcwhich agrees very well with the values derived from the model.The fact that we also find numerous embedded cYSOs in andnear the rim of the bubble is consistent with star-formationac-cording to the ‘collect and collapse’ model.

4.4.3. cYSOs in pillars

In the southern and eastern part of the bubble edge we find anumber of small pillars extending into the bubble interior.Theycan be seen in the optical and infrared in Fig. 2 and a closeupof the IR pillars is shown in Fig. 9. Within four of them wefind cYSOs in their very tips, reminiscent of what is observedinthe central Carina Nebula (e. g. the South Pillars region). One ofthem, J103806.6–584002, coincides with aHerschel point-likesource (cf. Sect. 5.1) and is therefore most probably a protostar.This suggests that radiative triggering is at work, very similarto the processes seen in the South Pillars (Smith et al. 2010b;Gaczkowski et al. 2013).

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Fig. 7. Herschel RGB image (red: 500µm,green: 250µm, blue: 70µm) with positions ofall Herschel point-like sources detected in atleast two bands overlaid. Red crosses showthe position of protostellar, yellow boxes thoseof prestellar cores. The large yellow rectanglemarks the borders of the region analysed.

4.4.4. G286.21+0.17

The cloud clump G286.21+0.17 ([DBS2003]127, BYF73) is lo-cated about 12′ north of the rim of the Gum 31 bubble. Thestructure of this clump was recently studied in several molec-ular lines by Barnes et al. (2010). They determined a diameterof ∼ 0.9 pc, a luminosity of∼ 2 – 3 · 104 L⊙, and estimateda clump mass of about 20 000M⊙ (a value that is about fortytimes larger than the previous mass estimate based on millime-tre data by Faundez et al. 2004). Their derived mass infall rateof ∼ 3.4 · 10−2 M⊙ yr−1 would be the highest mass infall rate yetseen, if confirmed. These properties make this cloud a particu-larly interesting site of possible massive-star formation.

In ourHerschel maps, this clump appears as a very bright andprominent compact feature. In Fig. 11, we compare its morphol-ogy in theHerschel far-IR bands to a near-IR image from ourVISTA data and aSpitzer mid-IR image. As already discussed byBarnes et al. (2010), a young stellar cluster, surrounded bydif-fuse nebulosity, is located immediately north-west of the clump.This cluster appears very prominent in Fig. 8 as well and con-tains∼ 45 cYSOs. In the centre of the clump itself, theSpitzerimages show two bright point sources with an angular separa-tion of 7.6′′ (Fig. 11). They can be identified with the 10µmpoint sources J103832.08–581908.9 and J103832.71–581914.8that were detected as counterparts of the MSX/RMS massivecYSO G286.2086+00.1694 described by Mottram et al. (2007),for which a bolometric luminosity of 7750L⊙ was determined.

The VISTAH-band image shows very faint diffuse nebulosi-ties at the location of these two mid-IR sources (see the close-upin Fig. 10). This is consistent with the idea that these two objectsrepresent deeply embedded YSOs in the protostellar evolution-ary phase. With WISE the two sources are not resolved but runinto one that we classify as a Class I candidate.

The peak of theHerschel PACS 70µm emission is centredon the mid-IR source J103832.0–581908. A two-dimensionalGaussian fit to the 70µm emission yields a FWHM size of13.6′′ × 11.3′′, which is clearly larger than the FWHM size of

10′′×10′′ measured for several isolated point-like sources in thesame map. With an angular distance of 7.6′′, the emission of thetwo mid-IR sources cannot be resolved in theHerschel maps, butthe measured direction of the elongation towards J103832.71–581914.8 suggests that both contribute to the observed far-IRemission.

A second cYSO is found in the south-east of the clump, atthe projection of the line connecting J103832.08–581908.9andJ103832.71–581914.8. It probably corresponds to the IRAC-identified source atα2000 = 10:38:33.6,δ2000 = −58:19:22.There is a third identified cYSO, situated at the very edge ofthe clump, atα2000 = 10:38:35.0,δ2000 = −58:18:44. Thesethree are the only cYSOs identified in the cluster with WISE,while in the immediately adjoining cluster of stars visiblein theIRAC images a large number of cYSOs is found. There are∼ 45overall, with a slight majority of Class II sources over Class Isources. If there is any trend in their spatial distribution, Class Iare found with slight emphasis to the south-east, while Class IIsources tend to be located towards the north-west.

Our colour-temperature map (Fig. 4a), constructed from theHerschel 70µm and 160µm maps, shows that the cloud temper-atures range from<∼ 20 K at the edge of the clump to∼ 25 – 30 Kin the clump centre, and up to 33 K in the nebulosity surroundingthe stellar cluster north-west of the clump.

In our Herschel column-density map (Fig. 4b), the levelNH = 2 · 1022 cm−2 traces the shape of the clump. This agreesvery well with the morphology as seen in the 1.2 mm map shownby Faundez et al. (2004). The peak value of the column densityis found to be 1.4 · 1023 cm−2 and corresponds to a visual extinc-tion of AV ≈ 70 mag. From our column density map we deter-mined the mass of the clump by integrating over a 200′′ × 200′′

(2.23 pc× 2.23 pc) box around the clump and subtracting thelocal background level. This yields a clump mass of 2 105M⊙.This value is nearly five times as large as the 470M⊙ derivedby Faundez et al. (2004), but a factor of ten smaller than the20 000M⊙ estimated by Barnes et al. (2010). This seems to sug-

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H. Ohlendorf et al.: Discovering young stars in the Gum 31 region with infrared observations

Fig. 8: Spitzer IRAC RGB image (red: 8.0µm, green: 4.5µm, blue: 3.6µm) with positions of WISE-identified cYSOs overlaid.Class I sources are marked by red diamonds, Class II in green.The white ellipses represent the clusters discussed in Sect. 4.4, withthe yellow circle marking the multiple star HD 92206 which contains the O stars in NGC 3324. The orange circles mark the cYSOclusters discussed in Sect. 4.4.6. The yellow rectangle marks the borders of the region analysed.

gest that the mass infall rate estimated by Barnes et al. (2010) isalso too high, and this clump is not as extreme as suspected.

Its far-IR fluxes as derived from ourHerschel data usingelliptical apertures for photometry with GAIA5 are 1661 Jy,2261 Jy, 1293 Jy, 653 Jy and 297 Jy for 70µm, 160µm, 250µm,350µm and 500µm, respectively, which results in an integratedfar-IR luminosity (70µm to 1.3 mm) ofLint ≈ 9000L⊙.

This is clearly one of the most luminous clumps in the CNC.Its mass is rather high, but probably not as high as previouslysuggested. It may form stars with<∼ 10M⊙, but is probably notmassive enough for the formation of high-mass stars withM∗ >∼20M⊙.

5 http://astro.dur.ac.uk/˜pdraper/gaia/gaia.html

4.4.5. G286.38–0.26

The Spitzer IRAC images show a prominent dense cluster ofseveral dozen stars at the southern edge of the Gum 31 bub-ble, which is surrounded by bright diffuse nebulous emission(Fig. 12). This cluster is listed as [DBS2003] 128 by Dutra etal.(2003). It is spatially coincident with the extended (r = 2 pc)C18O clump6 No. 6 (Yonekura et al. 2005).

The nebulosity around the stellar cluster displays a remark-able arc-like shape at the eastern edge. Projecting it into afull circle, it would have around 42′′ diameter in IRAC im-ages and 62′′ in Herschel images. The centre of this circle

6 We note that Yonekura et al. (2005) denoted these structuresas‘cores’; however, according to the definition thatcores are very com-pact clouds (with typical sizes of∼ 0.1 pc or less), out of which individ-ual stellar systems form, these clouds are better characterised asclumps(i. e. relatively large dense clouds linked to the formationof small stellarclusters).

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H. Ohlendorf et al.: Discovering young stars in the Gum 31 region with infrared observations

Fig. 9: In aSpitzer IRAC 8.0µm image, the yellow arrows mark the positions of four remarkable cYSOs found in the very tips ofpillars. Positions of WISE-identified cYSOs are overlaid asin Fig. 8: Class I sources are marked by red diamonds, Class IIsourcesin green. Blue crosses markHerschel point-like sources detected in at least two bands.

would be aroundα2000 = 10:38:03,δ2000 = −58:46:19. Thestar J10380461–5846233, cYSO [CNA2008] 21, is found close(∼ 11′′) to this central position of the arc. In theSpitzer bands,the cYSO shows strongly increasing brightness with wavelength.In the MIPS 24µm image, it is the brightest point-source in thecluster. It was detected as a mid-IR source by MSX and is listedas G286.3773-00.2563 in the MSX6C catalogue. With WISEdata we classify it as a Class II cYSO. The star is not detectedin any of ourHerschel far-IR images. Using USNO-B opticalcatalogue data, 2MASS, IRAC, MIPS, and WISE photometryand Herschel upper limits we employed the online SED fitterby Robitaille et al. (2007) to construct an SED and thus estimatethe (proto-) stellar parameters. The stellar mass is estimated to be≈ 5.8 M⊙ for the best-fit model, the luminosity≈ 238L⊙. Withinthe arc three further WISE Class I cYSOs are seen, J103805.8–584542, J103758.4–584648 and J103800.7–584654.

The optically brightest star in the cluster is HD 303094, forwhich a spectral type A2 is given by Nesterov et al. (1995). Itis

located about 17′′ south of J10380461–5846233 and the centreof the arc. According to the Pickles & Depagne (2010) survey ofall-sky spectrally matched Tycho-2 stars it may be a foregroundstar (distance: 886 pc).

Strong far-infrared emission from the region of this clusterwas detected with IRAS (point source IRAS 10361–5830). OurHerschel maps resolved this far-IR emission into ten point-likesources in the area of the clump. The onlyHerschel source withnear-IR counterpart is J103801.4–584641. The IRAC image atthis point is dominated by strong nebular emission. There isextended emission in a confined region a few arcseconds eastof the Herschel-identified point-like source, but since it is notwell-resolved and the identification with theHerschel-identifiedsource is not unambiguous, we do not include it in the sam-ple studied in Sect. 5. The twoHerschel-identified point-likesources north-east of the arc-like nebula that are also detectedas bright sources in the MIPS maps are J103810.2–584527 andJ103807.2–584511, both have no clear near-IR counterparts. A

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H. Ohlendorf et al.: Discovering young stars in the Gum 31 region with infrared observations

Fig. 11: Cluster G286.21+0.17 (red circle; cf. Sect. 4.4.4) and its immediate surroundings from near to far-IR. The two green circlesmark the two brightSpitzer-resolved sources within it. (A close-up of the VISTAH-band image is shown in Fig. 10.)

Fig. 10: Close-up of the VISTAH-band image (Fig. 11) aroundthe two bright IRAC sources (green circles) within the clusterG286.21+0.17.

similar case is J103754.0–584614, which has a very faint nebu-lous near-IR counterpart in the VISTA image. All three are also

detected as point-like sources in our IRAC images. They are in-cluded in Table A.1 (online material), but only for J103807.2–584511 we obtained the model parameters listed in Table A.2as for the others the quality of the SED fit was not sufficient.Additionally, all three sources described are classified asClass Isources from WISE data.

In the south-western part of this extended C18O clump,Yonekura et al. (2005) detected a compact (r = 0.27 pc) H13CO+

clump (their clump No. 2); with a central density ofn(H2) =6.8 · 104 cm−3 this is the densest of all the clumps they detectedin their survey of the Carina Nebula complex. TheHerschel-identified point-like source J103750.8–584718 coincides withthis clump. With IRAC, there are several small point-like sourcesseen to be coincident with it and an identification is therefore im-possible. An IRAC-detected source slightly to the south-west ofit, J103749.3–584722, is identified as a Class I WISE cYSO.

This area also contains the A0 supergiant HD 92207. Thisstar has a strong near-IR excess, was detected as a 12µmsource with IRAS (IRAS 10355–5828), and is seen as a point-like source surrounded by nebulosity in the MIPS 24µm image.Neither the star not the surrounding nebulosity can be seen in theHerschel images.

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H. Ohlendorf et al.: Discovering young stars in the Gum 31 region with infrared observations

Fig. 12: Cluster G286.38–0.26 from near- to far-infrared. The large red ellipse marks the approximate outline of the C18O clumpNo. 6 ([YAK2005] C18O 6; Yonekura et al. 2005). The blue circle marks H13CO+ clump No. 2 ([YAK2005] H13CO+ 2), the greencircle indicates the shape of the arc-like nebulosity visible in theSpitzer IRAC images.

4.4.6. Other structure

Towards the south-west of the Gum 31 shell, two reflection neb-ulae are found, GN 10.34.5 and GN 10.31.8 (both are markedwith white ellipses in Fig. 8). Both are very clearly delineatedby cYSOs and coincide with two of the most conspicuous clus-ters in our field of view. GN 10.34.5 is visible in theSpitzer RGBimage (especially the 4.5µm band) and seen in projection witha dozen cYSOs. GN 10.31.8, on the other hand, is both largerin angular extent and coincident with four times the number ofcYSOs. It, too, is very conspicuous in the 4.5µm band.

There also is a smaller cluster of about two dozen IRACpoint-like sources centred atα2000 = 10:38:03, δ2000 =

−58:55:09 within the Gum 31 shell (orange circle in Fig. 8).Immediately to its west there lies molecular cloud No. 7 ofYonekura et al. (2005). It is accompanied by emission visiblein the Herschel bands and a peculiarly green point-like featurein the IRAC RGB image, that is, strong emission in the IRAC4.5µm band.

In the distribution of IRAC-identified cYSOs there is an-other notable cluster consisting of 12 candidates to the west ofthe H II region aroundα2000 = 10:34:27,δ2000 = −58:46:45(orange circle in Fig. 8). This region is devoid of mid-IR emis-sion but is coincident with C18O clump No. 1 of Yonekura et al.(2005) and far-IR emission as traced byHerschel. The cYSOsare aligned along the western ridge of the far-IR cloud as seenin theHerschel image and even follow the shape of its filaments,broadly in the shape of an arrowhead pointing eastwards. Thenorthern part is better aligned with the filament shape whilethesouthern part is more randomly distributed around the filament

itself. The border is also traced by the Yonekura et al. (2005)12CO intensity contours. With the WISE classification, however,there is nothing remarkable about that region. We find Class Icandidate J103423.7–584531 to the north and Class II candidateJ103424.6-584749 to the south, but no appearance of clustering.

North of the Gum 31 bubble, aroundα2000 = 10:37:36,δ2000 = −58:26:36, there is a cluster of stars clearly discerniblein the IRAC image (orange circle in Fig. 8). It lies about 2′

to the south-west of (Yonekura et al. 2005) C18O clump No. 4.Around three dozen stars are seen within this group in pro-jection and it is also associated with a number ofHerschel-identified point-like sources. Two of them, J103739.6–582756and J103741.7–582629, are part of the sample analysed inSect. 5. Few stars within or around this cluster are identified ascYSOs with WISE. J103736.3–582655 and J103741.9–582556are the brightest stars in the IRAC images of the cluster and bothidentified as Class I candidates with WISE. There is one moreClass I candidate and five Class II candidates distributed fairlyevenly over the cluster.

There are several more minor clusters of∼ 5 cYSOs, no-tably always coincident with local maxima in the Yonekura etal.(2005) C18O maps.

5. SED modelling for sources with both Herscheland Spitzer counterparts

Using ourHerschel andSpitzer catalogues, we were able to iden-tify those sources that are detected as point-like sources in bothwavelength ranges. ForHerschel we applied the restriction that

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H. Ohlendorf et al.: Discovering young stars in the Gum 31 region with infrared observations

the sources had to be detected in at least three of the five bands,bringing the total number down from 91 sources detected in atleast two bands to 59. This results in 16 sources overall thatcan be identified in at least threeHerschel bands and at leastone of the IRAC bands. We then compared these identificationsto the MIPS image and performed photometry as described inSect. 2.1.2 for those 6 sources where we could identify a MIPScounterpart.

To extend the wavelength range of our observations, we ad-ditionally matched the point sources analysed here with sourcesfrom the 2MASS (Skrutskie et al. 2006) All-Sky Catalog ofPoint Sources (Cutri et al. 2003). This was performed applyingthe same next-neighbour search as for the inter-band matchingwithin the IRAC sources (Sect. 2.1.1), using only sources withquality flags A to D. We then repeated this process with theWISE catalogue, where we selected only those sources that hada signal-to-noise ratio larger than 5. A detailed overview of allphotometric data assembled is given in Table A.1 in the onlinematerial. WISE 22µm and MIPS 24µm fluxes show some in-congruity, however there is no underlying pattern as to whatinthe environs of the source may have influenced the photometry.This does not, however, influence the findings from the SED fits.In the two cases where the 22µm flux appears unduly high com-pared to the 24µm flux and an SED fit is performed, leavingout one or the other from the fit has little or no influence on thebest-fit model.

5.1. Modelling of the SEDs

For SED-fitting we used the online tool of Robitaille et al.(2007). This tool compares the input observational data with200 000 SED models for YSOs that were precomputed using a2D radiative transfer code by Whitney et al. (2003). These mod-els have a wide parameter space for the properties of the centralobject and its environment.

For the fits, the distance to all objects was assumed to be2.3 kpc (cf. Sect. 3), and the interstellar extinction rangewas re-stricted toAV = 0 . . .40 mag. We assumed an uncertainty of 20%for all Herschel fluxes. For 2MASS,Spitzer and WISE fluxes inaddition to the individual photometric measurement uncertaintyas given in Table A.1 we assumed a further systematic uncer-tainty of 10% due e. g. to the reliability of flux calibration.ForIRAC, photometry varies by up to 10% due to the position of thepoint-like source within the detector array and though appropri-ate corrections were applied in the process, this is an additionalsource of uncertainty.

In addition to the best-fit model, we show the range of possi-ble parameters that can be derived from models within the rangeof χ2/N − χ2

best/N < 2 (with N representing the number of datapoints). These models are shown as grey lines in the plots inFig. 13. The resulting model parameters are listed in Table A.2.It gives the best-fit value together with the range constrained bythe aboveχ2 criterion. The resulting SEDs are shown in Fig. 13.We only use fits whereχ2/N for the best-fit model is smaller thanor equal to 10.0.

5.2. Results of SED modelling

The results of SED fits can be highly ambiguous. Many of thestellar and circumstellar parameters are often poorly constrainedbecause the models show a high degree of degeneracy (e. g.Men’shchikov & Henning 1997). We therefore restrict our anal-ysis to a few selected parameters that can be relatively welldeter-

mined from these fits. These are the total luminosity, the stellarmass, and the mass of the circumstellar disk and envelope.

The best-fit masses as listed in Table A.2 are between 1.7M⊙and 6.6M⊙ and even the extremes of the ranges do not exceed1.2M⊙ to 7.1M⊙. The majority of luminosities are to be found ina range of∼ 100 – 300L⊙, with two notable exceptions well be-low that at 38L⊙ and 42L⊙, respectively, and one exceptionallyluminous source at 890L⊙ best-fit value, corresponding with thehighest best-fit stellar mass in our sample. Whereas 4 of the 10sources sampled here exhibit best-fit envelope masses of 190M⊙or higher, 3 are at≤ 100M⊙ and two lower than 50M⊙. The diskmasses span a range of about one order of magnitude between∼ 0.01M⊙ and∼ 0.1 M⊙. The highest-mass star in the sample isthe notable exeption with a disk mass of∼ 0.001M⊙.

In a large-scale view it is immediately noticeable that all buttwo of the sources for whichHerschel counterparts toSpitzerpoint-like sources are detected are to be found within the Gum 31bubble. Although the field of our study stretches further outespe-cially to the west, only two sources are found outside the bubble.These are J103557.6–590046 and J103427.3–584611.

J103806.6–584002 is remarkable in that contrary to the vastmajority of objects it is not located in the rim of the bubble butwithin the bubble itself, being the only specimen in our sample.It is placed in the very head of a pillar-like filament that extendsfrom the northern rim of the bubble into it (cf. Fig. 9). With themethods employed in Sect. 4 we classify it as a WISE Class Icandidate and an IRAC cYSO. In theHerschel images the fila-ment is rather faint, but J103806.6–584002 itself is clearly visi-ble as a point-like source.

6. Sources of HH jets

In an earlier paper (Ohlendorf et al. 2012) we have traced a num-ber of Herbig-Haro jets identified in the Carina Nebula com-plex by Smith et al. (2010a) back to their protostellar sources.Smith et al. (2010a) also identify two HH jets and two HH jetcandidates in the Gum 31 bubble. It should be remarked that theHST images on which the Smith et al. (2010a) study is basedcover only a very small area of the entire Gum 31 region andthe following analysis only represents a very small sectionofthe area covered in the rest of the paper. Due to the small sam-ple size and its very limited spatial dimensions, we cannot drawany conclusions about the distribution or likely number of alljet-driving protostars within the Gum 31 region.

As can be seen in Fig. 14, the jet HH 1002 is almost per-pendicular to the ionisation front. It shows a number of dis-tinct features, marked by Smith et al. (2010a) as A, B and C andlabelled thus in our figure. Smith et al. (2010a) remark that inthe 2MASS images they detect a very likely source located atα2000= 10:36:53.9,δ2000= −58:37:19. This matches almost ex-actly with the location of the object identified in IRAC imagesthat is most likely to be the source of the jet: J103654.0–583720,which we classified as being a cYSO through its WISE andIRAC infrared excesses in Sect. 4.1. TheHerschel observationstrace the ionisation front well, but show no point-like source tobe coincident with J103654.0–583720. Therefore it could not beincluded in the SED-based analysis in Sect. 5.

HH 1003 has a more complicated structure and Smith et al.(2010a) discuss it as possibly being a two-part object, madeoutof two jets in close association. Tracing back the directionofthe bow shocks in Fig. 14, we find two point-like sources in theIRAC images that are very probably the sources of two differentjets. One of them, J103652.4–583809,associated with features A

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Fig. 13: Spectral energy distributions of those objects forwhich we could determine fluxes with at least threeHerschel and oneSpitzer IRAC band. Filled circles mark the input fluxes. The black line shows the best fit, and the grey lines show subsequent goodfits. The dashed line represents the stellar photosphere corresponding to the central source of the best fitting model, asit wouldappear in the absence of circumstellar dust (but including interstellar extinction).

and B, is identified as a cYSO from the WISE and IRAC colour-colour diagrams, while the other is an IRAC cYSO. Both arecoincident with faint sources in the 2MASS images. Again, theHerschel image shows no point-like sources.

The two candidate outflows, HH c-1 and HH c-2 (not tobe confused with HH c-1 and HH c-2 within the extent of thecentral Carina Nebula which were included in the study ofOhlendorf et al. 2012), could not be traced back to any IRsources within the scope of our study. Following the axes of thejets, we cannot identify any likely emitting sources withintheirimmediate surroundings.

7. Discussion and Conclusions

In this paper we analysedSpitzer, WISE, andHerschel data toinvestigate the cloud structure and the young stellar populationin and around the Gum 31 nebula. These data provide consider-ably better sensitivity and spatial resolution than the previouslyavailable data sets.

TheHerschel far-IR maps show that the bubble surroundingthe Gum 31 nebula is connected to the central parts of the CarinaNebula. This adds strong direct support to the assumption that

Gum 31 is actually part of the Carina Nebula complex, as is alsosuggested by the matching C18O radial velocities measured byYonekura et al. (2005) and other recent distance determinations(cf. Sect. 1).

The bubble itself has a very sharp western edge, wherethe column density derived fromHerschel measurements risesabruptly by at least an order of magnitude. The dust tempera-tures range from<∼ 20 K in the dark clouds surrounding the H IIregion to≈ 30 K in the H II region and up to≈ 40 K near thelocation of the O-type stars.

The very small excess fraction seen for the mid-IR sourcesin the central stellar cluster NGC 3324 (≈ 0.5%) suggests that itis at least several Myr old already. For the whole Gum 31 region,theSpitzer and WISE data reveal about 300/660 cYSOs. Theseobjects are most likely Class I protostars or Class II sources ofsolar to intermediate mass. The 59 far-IR point-like sources re-vealed by theHerschel data are either pre-stellar cores or em-bedded (Class 0) protostars, i. e. trace a younger population ofcurrently forming stars. The spatial distribution of the cYSOs ishighly non-uniform. As we expect a contamination of around18% most of the widely distributed YSO population we seeis probably due to back- and foreground stars as those would

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Fig. 14: The Herbig-Haro jets and their probable sources as seen in four different wavelengths. From left to right: Hubble SpaceTelescope RGB image (red: WFPC2 S II filter (673 nm), green: ACS Hα +N II filter (658 nm), blue: WFPC2 O III filter (502 nm);image credit: NASA/ESA/Hubble Heritage Team (STScI/AURA)), Spitzer IRAC RGB image with 3.6µm in blue, 4.5µm in greenand 8.0µm in red,Herschel PACS 170µm image and VISTAH-band image. The yellow or blue arrows indicate the broad shape anddirection of the outflow in question, white letters mark their features as described by Smith et al. (2010a). The probableIR-identifiedsources are marked at theirSpitzer coordinates with circles in yellow or blue.

be expected to be distributed homogeneously. Many cYSOs arefound in rather compact clusterings, and a considerable numberis found at the inner edge of the dusty bubble surrounding theH II region. TheHerschel-identified point-like sources in partic-ular trace the edge of the bubble.

This led us to assume a ‘collect and collapse’ scenario drivenby the O stars within the cluster (Whitworth et al. 1994), result-ing in an expected bubble size of 9 – 11 pc for an age of 1.5 –2.0 Myr. This agrees very well with the∼ 10 pc diameter ob-served. We find four cYSOs in the very tips of small pillars ex-tending from the bubble rim (cf. Fig. 9), suggesting radiativetriggering processes very similar to what is observed in theSouthPillars (Smith et al. 2010b; Gaczkowski et al. 2013).

We conclude that two different modes of triggered star-formation occur simultaneously in the Gum 31 region: ‘collectand collapse’, as evidenced by the bubble size and the cYSOs inits rim, and radiative triggering, as evidenced by cYSOs in theheads of pillars.

We construct the near- to far-infrared SEDs of 17 cYSOsand estimate basic stellar and circumstellar parameters bycom-parison to radiative-transfer models with good-quality fits for 10of them. All these cYSO s are of moderate luminosity (L <∼900L⊙), clearly suggesting that they are low- or intermediate-mass objects (M <∼ 7 M⊙). This agrees with the results from ouranalysis of the cYSOs in the central parts of the Carina Nebulacomplex (Gaczkowski et al. 2013), where we found that no high-mass stars are currently forming.

We identify the driving sources of two Herbig-Haro jetsin the western rim of the Gum 31 bubble. These sources arealso identified as cYSOs by applying colour-selection criteriato IRAC or WISE photometry data.

From the total number of cYSOs observed and the IMF(Kroupa 2002) we can estimate a total young stellar populationfor the Gum 31 region. Our detection limit for cYSOs is about

1 M⊙ and following the IMF there should be eight times as manystars below this mass (> 0.1 M⊙) as above it. Correcting thenumber of cYSOs given in Sect. 4.2 for the contamination esti-mated there, this gives a number of∼ 5000 young stellar objectsin the region.

A more detailed investigation of the star formation historyin this area requires a reliable identification of the individualyoung stars. While the infrared data presented in this papercanreveal protostars and young stars with circumstellar disks, mostof the slightly older (& 2 Myr old) stars cannot be detectedby infrared excesses. Our very recentChandra X-ray observa-tions of the Gum 31 region, near-IR observations with VISTAand other ongoing observations will allow us to finally identifythese stars, too. This will constitute the basis for a comprehen-sive multi-wavelength study of this interesting region, ina waysimilar to the recent studies of the young stellar populationsin the central parts of the Carina Nebula (see Townsley et al.2011; Preibisch et al. 2011a; Wang et al. 2011; Wolk et al. 2011;Feigelson et al. 2011).

Acknowledgements. This work was supported by the GermanDeutsche For-schungsgemeinschaft, DFG project number 569/9-1. Additional support camefrom funds from the Munich Cluster of Excellence “Origin andStructure of theUniverse”.

The authors would like to thank theSpitzer Science Center Helpdesk forlarge amounts of support while working with MOPEX.

This work is based in part on archival data obtained with theSpitzerSpace Telescope, which is operated by the Jet Propulsion Laboratory, CaliforniaInstitute of Technology under a contract with NASA.

This publication makes use of data obtained with theHerschel spacecraft.TheHerschel spacecraft was designed, built, tested, and launched undera con-tract to ESA managed by theHerschel/Planck Project team by an industrial con-sortium under the overall responsibility of the prime contractor Thales AleniaSpace (Cannes), and including Astrium (Friedrichshafen) responsible for thepayload module and for system testing at spacecraft level, Thales Alenia Space(Turin) responsible for the service module, and Astrium (Toulouse) responsiblefor the telescope, with in excess of a hundred subcontractors.

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This publication makes use of data products from the Two Micron AllSky Survey, which is a joint project of the University of Massachusetts andthe Infrared Processing and Analysis Center/California Institute of Technology,funded by the National Aeronautics and Space Administration and the NationalScience Foundation.

This publication makes use of data products from the Wide-field InfraredSurvey Explorer, which is a joint project of the University of California, LosAngeles, and the Jet Propulsion Laboratory/California Institute of Technology,funded by the National Aeronautics and Space Administration.

The science data reduction for VISTA up to the creation of thefinal tile wasperformed by the Cambridge Astronomy Survey Unit.

The Digitized Sky Survey was produced at the Space TelescopeScienceInstitute under U.S. Government grant NAG W-2166. The images of thesesurveys are based on photographic data obtained using the Oschin SchmidtTelescope on Palomar Mountain and the UK Schmidt Telescope.The plates wereprocessed into the present compressed digital form with thepermission of theseinstitutions.

This research has made use of NASA’s Astrophysics Data SystemBibliographic Services.

This research has made use of the SIMBAD database and the VizieR cata-logue access tool, operated at CDS, Strasbourg, France.

ReferencesAllen, L. E., Calvet, N., D’Alessio, P., et al. 2004, ApJS, 154, 363Baraffe, I., Chabrier, G., Allard, F., & Hauschildt, P. H. 1998, A&A, 337, 403Barnes, P. J., Yonekura, Y., Ryder, S. D., et al. 2010, MNRAS,402, 73Baumgardt, H., Dettbarn, C., & Wielen, R. 2000, A&AS, 146, 251Cappa, C., Niemela, V. S., Amorın, R., & Vasquez, J. 2008, A&A, 477, 173Carraro, G., Patat, F., & Baumgardt, H. 2001, A&A, 371, 107Claria, J. J. 1977, A&AS, 27, 145Cutri, R. M. & al. 2012, VizieR Online Data Catalog, 2311Cutri, R. M., Skrutskie, M. F., van Dyk, S., et al. 2003, VizieR Online Data

Catalog, 2246Cutri, R. M., Wright, E. L., Conrow, T., et al. 2012, Explanatory Supplement to

the WISE All-Sky Data Release Products, Tech. rep.Dalton, G. B., Caldwell, M., Ward, A. K., et al. 2006, in Society of Photo-Optical

Instrumentation Engineers (SPIE) Conference Series, Vol.6269Dewangan, L. K., Ojha, D. K., Anandarao, B. G., Ghosh, S. K., &Chakraborti,

S. 2012, arXiv:1207.6842v1Diolaiti, E., Bendinelli, O., Bonaccini, D., et al. 2000, A&AS, 147, 335Dullemond, C. P. & Dominik, C. 2004, A&A, 417, 159Dutra, C. M., Bica, E., Soares, J., & Barbuy, B. 2003, A&A, 400, 533Emerson, J. & Sutherland, W. 2010, The Messenger, 139, 2Fang, M., van Boekel, R., King, R. R., et al. 2012, A&A, 539, A119Faundez, S., Bronfman, L., Garay, G., et al. 2004, A&A, 426,97Fazio, G. G., Hora, J. L., Allen, L. E., et al. 2004, ApJS, 154,10Feigelson, E. D., Getman, K. V., Townsley, L. K., et al. 2011,ApJS, 194, 9Flaherty, K. M., Pipher, J. L., Megeath, S. T., et al. 2007, ApJ, 663, 1069Forte, J. C. 1976, A&AS, 25, 271Gaczkowski, B., Preibisch, T., Ratzka, T., et al. 2013, A&A,549, A67Griffin, M. J., Abergel, A., Abreu, A., et al. 2010, A&A, 518, L3Hoffleit, D. 1953, Annals of Harvard College Observatory, 119, 37Irwin, M. J., Lewis, J., Hodgkin, S., et al. 2004, in Society of Photo-Optical

Instrumentation Engineers (SPIE) Conference Series, ed. P. J. Quinn &A. Bridger, Vol. 5493, 411–422

Kharchenko, N. V., Piskunov, A. E., Roser, S., Schilbach, E., & Scholz, R.-D.2005, A&A, 438, 1163

Koenig, X. P., Leisawitz, D. T., Benford, D. J., et al. 2012, ApJ, 744, 130Kramer, C., Cubick, M., Rollig, M., et al. 2008, A&A, 477, 547Kroupa, P. 2002, Science, 295, 82Kudritzki, R. P., Puls, J., Lennon, D. J., et al. 1999, A&A, 350, 970Lada, C. J. 1987, in IAU Symposium, Vol. 115, Star Forming Regions, ed.

M. Peimbert & J. Jugaku, 1–17Lyngå, G. & Hansson, N. 1972, A&AS, 6, 327Maız-Apellaniz, J., Walborn, N. R., Galue, H.A., & Wei, L. H. 2004, ApJS, 151,

103Makovoz, D. & Marleau, F. R. 2005, PASP, 117, 1113Men’shchikov, A. B. & Henning, T. 1997, A&A, 318, 879Molinari, S., Schisano, E., Faustini, F., et al. 2011, A&A, 530, A133Mottram, J. C., Hoare, M. G., Lumsden, S. L., et al. 2007, A&A,476, 1019Nesterov, V. V., Kuzmin, A. V., Ashimbaeva, N. T., et al. 1995, A&AS, 110, 367Ohlendorf, H., Preibisch, T., Gaczkowski, B., et al. 2012, A&A, 540, A81Oliveira, I., Merın, B., Pontoppidan, K. M., et al. 2009, ApJ, 691, 672Pickles, A. & Depagne,E. 2010, PASP, 122, 1437Poglitsch, A., Waelkens, C., Geis, N., et al. 2010, A&A, 518,L2Preibisch, T., Hodgkin, S., Irwin, M., et al. 2011a, ApJS, 194, 10

Preibisch, T., Ratzka, T., Kuderna, B., et al. 2011b, A&A, 530, A34Preibisch, T., Roccatagliata, V., Gaczkowski, B., & Ratzka, T. 2012, A&A, 541,

A132Preibisch, T., Schuller, F., Ohlendorf, H., et al. 2011c, A&A, 525, A92Przybilla, N., Butler, K., Becker, S. R., & Kudritzki, R. P. 2006, A&A, 445, 1099Ragan, S., Henning, T., Krause, O., et al. 2012, A&A, 547, A49Rieke, G. H., Young, E. T., Engelbracht, C. W., et al. 2004, ApJS, 154, 25Robitaille, T. P., Whitney, B. A., Indebetouw, R., & Wood, K.2007, ApJS, 169,

328Salatino, M., de Bernardis, P., Masi, S., & Polenta, G. 2012,ApJ, 748, 1Skrutskie, M. F., Cutri, R. M., Stiening, R., et al. 2006, AJ,131, 1163Smith, N. 2006a, MNRAS, 367, 763Smith, N. 2006b, MNRAS, 368, 1983Smith, N. 2006c, ApJ, 644, 1151Smith, N., Bally, J., & Walborn, N. R. 2010a, MNRAS, 405, 1153Smith, N. & Brooks, K. J. 2007, MNRAS, 379, 1279Smith, N. & Brooks, K. J. 2008, Handbook of Star Forming Regions, Volume II,

ed. Reipurth, B., 138Smith, N., Povich, M. S., Whitney, B. A., et al. 2010b, MNRAS,406, 952Townsley, L. K., Broos, P. S., Corcoran, M. F., et al. 2011, ApJS, 194, 1Wang, J., Feigelson, E. D., Townsley, L. K., et al. 2011, ApJS, 194, 11Whitney, B. A., Wood, K., Bjorkman, J. E., & Wolff, M. J. 2003, ApJ, 591, 1049Whitworth, A. P., Bhattal, A. S., Chapman, S. J., Disney, M. J., & Turner, J. A.

1994, A&A, 290, 421Winston, E., Megeath, S. T., Wolk, S. J., et al. 2007, ApJ, 669, 493Wolk, S. J., Broos, P. S., Getman, K. V., et al. 2011, ApJS, 194, 12Wright, E. L., Eisenhardt, P. R. M., Mainzer, A. K., et al. 2010, AJ, 140, 1868Yonekura, Y., Asayama, S., Kimura, K., et al. 2005, ApJ, 634,476

Appendix A: Online material

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observationsTable A.1: Source fluxes as obtained in theSpitzer (IRAC: 3.6µm, 4.5µm, 5.8µm, and 8.0µm; MIPS: 24µm) andHerschel (PACS: 70µm and 160µm, SPIRE: 250µm,350µm, and 500µm) bands, complemented with WISE photometry from the All-Sky Data Release (3.4µm, 4.6µm, 12µm, and 22µm) and JHKs photometry as obtained fromthe 2MASS All-Sky Catalog of Point Sources.

Source FJ FH FKs F3.4µm F3.6µm F4.5µm F4.6µm

[mJy] [mJy] [mJy] [mJy] [mJy] [mJy] [mJy]

J103427.3–584611 – – – 0.939±0.015 0.902±0.033 2.395±0.033 5.173±0.023J103557.6–590046 8.624±0.069 13.35±0.11 12.380±0.082 16.095±0.061 8.902±0.037 – 15.002±0.051J103643.2–583158 – – 0.796±0.025 – 1.585±0.026 1.226±0.030 –J103645.9–584258 2.245±0.019 2.112±0.018 1.352±0.026 – – 0.336±0.034 –J103652.4–583129 – – 0.843±0.030 4.629±0.046 14.790±0.030 67.070±0.030 64.47±0.19J103700.9–583237 – – – 8.596±0.067 5.908±0.030 17.420±0.026 26.02±0.10J103703.6–584751 0.711±0.020 2.610±0.036 5.281±0.036 5.753±0.077 20.540±0.029 28.310±0.031 10.379±0.070J103726.7–584809 – – – – – 0.359±0.024 –J103737.3–584700 – 0.580±0.021 0.873±0.022 – 1.925±0.026 6.835±0.029 –J103739.6–582756 – – – – – 3.464±0.026 4.411±0.026J103741.7–582629 – – – – – 0.659±0.030 –J103754.0–584614 – – 0.876±0.031 5.03±0.15 9.175±0.029 44.960±0.030 67.63±0.24J103804.9–585533 – – – 30.11±0.17 50.610±0.024 158.300±0.033 211.13±0.52J103806.6–584002 – 1.119±0.033 4.066±0.039 8.316±0.047 9.277±0.024 13.320±0.035 12.363±0.059J103807.2–584512 – – 8.309±0.065 92.87±0.31 82.700±0.025 129.100±0.032 186.10±0.46J103810.2–584527 – – – 7.343±0.073 12.330±0.024 50.940±0.029 50.23±0.16J103842.1–584437 – 2.376±0.026 12.26±0.074 51.46±0.19 – 68.850±0.030 112.14±0.29

Notes. The given uncertainties are the individual photometric measurement uncertainties only, as derived (for IRAC and MIPS)or as obtained from the catalogue (for 2MASS and WISE). ForHerschel we did not obtain an uncertainty from photometry, but use an estimated total uncertainty of 20%.

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SourceF5.8µm F8.0µm F12µm F22µm F24µm F70µm F160µm F250µm F350µm F500µm

[mJy] [mJy] [mJy] [mJy] [mJy] [Jy] [Jy] [Jy] [Jy] [Jy]

J103427.3–584611 2.84±0.13 2.21±0.11 – 188.04±0.42 – 15.8 11.0 10.7 12.2 4.47J103557.6–590046 – 70.36±0.13 177.29±0.27 344.38±0.46 – 4.03 7.63 6.09 – –J103643.2–583158 – – – – – – 22.6 14.4 21.2 –J103645.9–584258 – – – – – – – 9.50 8.54 2.69J103652.4–583129 162.60±0.12 197.00±0.11 77.32±0.39 623.2±1.9 755.35±0.14 21.3 13.9 11.2 – 11.2J103700.9–583237 – – 486.49±0.48 1171.2±1.3a 395.42±0.14 – 19.1 19.9 11.2 6.96J103703.6–584751 35.55±0.15 25.51±0.13 – 257.42±0.61 181.70±0.13 1.32 6.72 10.8 6.32 2.89J103726.7–584809 – – – – – – 12.2 11.6 9.68 7.10J103737.3–584700 17.14±0.14 27.08±0.11 – 631.3±2.6 149.17±0.14 – 31.4 28.1 29.8 18.4J103739.6–582756 7.78±0.13 5.47±0.13 – 164.83±0.38 245.81±0.14 – 7.68 14.1 12.0 5.68J103741.7–582629 – – – – – – 11.1 11.4 9.05 6.68J103754.0–584614 113.80±0.13 135.80±0.13 134.2±1.4 1146.7±2.4 858.43±0.14 9.03 35.8 79.6 40.7 32.4J103804.9–585533 309.70±0.14 364.000±0.099 81.48±0.80 2197.1±1.2 – 5.91 19.8 11.8 9.01 6.23J103806.6–584002 17.23±0.12 18.75±0.14 60.29±0.34 158.6±2.2 – – 10.6 13.1 5.70 2.09J103807.2–584512 286.30±0.15 526.00±0.14 664.2±1.1 2409.2±1.9 1436.24±0.14 15.2 35.1 26.8 – –J103810.2–584527 151.40±0.13 245.00±0.15 276.9±1.1 843.8±1.9 1814.13±0.14 6.32 23.6 43.0 46.2 54.2J103842.1–584437 111.60±0.11 146.40±0.14 269.07±0.88 714.2±1.4 505.00±0.13 24.1 14.8 15.4 10.2 5.63

Notes. The given uncertainties are the individual photometric measurement uncertainties only, as derived (for IRAC and MIPS)or as obtained from the catalogue (for 2MASS and WISE). ForHerschel we did not obtain an uncertainty from photometry, but use an estimated total uncertainty of 20%.(a) The extremely high 22µm flux compared to the 24µm flux is probably due to thesources close proximity to a much brighter point source the contribution of which might not fully have been removed in theWISE All-Sky Data Release.

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Source Stellar mass Disk mass Envelope mass Total luminosity Best-fit χ2

N[M⊙] [ M⊙] [ M⊙] [ L⊙] model

J103427.3–584611 5.8 [5.8 – 7.3] 1.02·10−1 [4.30·10−3 – 1.02·10−1] 110 [110 – 170] 320 [320 – 400] 3003596 6.9J103557.6–590046 1.7 [1.2 – 2.0] 3.91·10−2 [1.76·10−3 – 3.91·10−2] 27 [4.2 – 60] 42 [27 – 68] 3015149 5.5J103645.9–584258 4.2 [3.9 – 5.2] 9.61·10−2 [1.09·10−3 – 9.61·10−2] 46 [18 – 46] 160 [140 – 190] 3006782 8.2J103726.7–584809 6.0 [2.6 – 6.0] 2.64·10−2 [1.05·10−3 – 1.53·10−1] 310 [160 – 310] 190 [94 – 190] 3008699 2.6J103739.6–582756 5.8 [3.9 – 6.9] 2.92·10−2 [2.21·10−3 – 2.16·10−1] 190 [140 – 410] 240 [160 – 360] 3009009 4.8J103741.7–582629 3.4 [2.6 – 6.0] 9.53·10−2 [1.62·10−3 – 1.53·10−1] 250 [160 – 310] 94 [94 – 190] 3005296 2.8J103806.6–584002 1.7 [1.7 – 2.6] 4.57·10−2 [1.34·10−2 – 4.57·10−2] 120 [52 – 120] 38 [38 – 58] 3016199 8.8J103807.2–584512 6.6 [1.7 – 7.3] 6.77·10−4 [6.77·10−4 – 5.06·10−1] 130 [5.3 – 310] 890 [220 – 890] 3010777 7.8J103842.1–584437 3.5 [1.7 – 3.9] 1.03·10−2 [6.45·10−3 – 1.97·10−1] 220 [13 – 220] 170 [130 – 190] 3011717 6.6

Notes. For every model parameter the best-fit-value is given in the respective first column, followed by a range defined by the minimum and maximum value obtained from models constrained by aχ2 criterion. The second-to-last and last columns give the identifier of the best-fit model and itsχ2/N (with N representing the number of data points).

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